Exercise treadmills allow people to walk, jog, run, and sprint on a stationary machine with an endless belt moving over a front and rear sets of pulleys.
Arrays of rollers have been used to support objects so they can be moved linearly with low friction. The minimum distance between the roller axes necessarily must be greater than the diameter of the roller. This leaves an undesirable distance from the top of one roller to the next in supporting an object. To overcome this, the array of rollers for such support applications has been replaced by a nested array of casters or wheels where the wheels on adjacent axes are offset laterally so that support distances from the top of one wheel to the next is smaller than that of adjacent rollers of similar diameter. The patent of Janitsch (U.S. Pat. No. 4,195,724) shows a similar technique in staggered rollers in a conveying elevator for granular material. The patent of Kornylak (U.S. Pat. No. 3,964,588) for a manual box conveyor illustrates the use of wheel arrays partially nested in several embodiments.
In the design of treadmills using rollers to support a lightweight belt along the length of a concave top surface, the problem of the upper belt surface lifting up away from the support rollers and presenting a flattened appearance has been addressed by the US patent application 2012/0157267 of Lo by the use of an array of guiding elements on either side of the belt in contact with the upper face of the upper concave surface. These elements are small wheels which physically extend above the belt surface to hold it down against the underlying rollers.